Amorphous semiconductor films having a sufficiently small density of localized states have been recognized as having many potential applications, including in photovoltaic devices. Hydrogen and fluorine have been recognized as effective in reducing the density of localized states, i.e., passivating, amorphous silicon films. With respect to hydrogen passivation of amorphous silicon, it has been found that the particular coordination of the hydrogen atoms with silicon atoms in hydrogenated amorphous silicon films (a-Si:H) has a dramatic effect on the electronic properties of those films. Films having the most desired electronic characteristics have large silicon monohydride concentrations relative to the concentration of polyhydrides. In fact, for electronic device quality material it is desired that only silicon monohydrides be present and that silicon polyhydrides be entirely absent from a-Si:H films.
Typically, hydrogenated amorphous silicon has a measured band gap energy of 1.7 to 1.8 eV meaning no electrical charge carriers can be generated in those films by photons having an energy less than that band gap energy. Therefore a significant amount of solar energy cannot be converted to electrical energv by amorphous, hydrogenated silicon films. On the other hand, if the band gap energv of a film is too low, the efficiency of the semiconductor film in generating charge carriers in response to incoming photons is reduced. Various semiconductor materials have band gap energies lower than that of amorphous silicon, but are unsuited for photovoltaic cells for numerous reasons, including the difficulty and expense of preparation. An ideal band gap energy, compromising efficiency of charge carrier generation and responsiveness to the energy range of photons present in solar illumination, is about 1.4 eV.
lt is known that alloys of elemental semiconductors having band gap energies intermediate those of the constitutent elements can be prepared. For example, single crystal alloys of germanium and silicon have band gap energies between their elemental values (0.7 eV and 1.1 eV, respectively) depending upon the relative proportions of germanium and silicon in the alloy. The same band gap energy grading observed in crystalline alloys occurs in passivated amorphous compound semiconductor films, but amorphous semiconductor alloy films having satisfactory electronic properties have been difficult to prepare.
Numerous processes have been employed to produce a-Si:H films. Among the deposition techniques that have been used are glow discharge and radio frequency and ion beam sputtering. An example of ion beam sputtering using a single ion gun is disclosed in U.S. Pat. No. 4,376,688 to Ceasar et al. There, a mixed beam of hydrogen and argon ions produced by a single ion source bombarded a silicon target to produce a hydrogenated silicon film relatively free of polyhydrides. The single ion beam energy and current of the mixed ion beam limit the sputtering efficiency of that process for any particular degree of hydrogenation. For example, when the hydrogen concentration in the single ion beam source was increased from zero to 75 percent to achieve film hydrogenation, the rate of film deposition fell from 3 micrometers per hour to 0.6 micrometers per hour. Dual ion beam sputtering was reported by Coluzza and others in an article "a-Si:H Produced by Double Ion-Beam Sputtering" in 59 and 60 Journal of Non-Crystalline Solids, 723-726 (1983). In that article, the authors disclosed directing a sputtering ion beam (argon) at a silicon target and a passivating ion beam (hydrogen) at the substrate on which the sputtered amorphous silicon was condensed. No improvement was achieved in controlling the amount and form of hydrogen content of the film deposited by the Coluzza et al. dual ion beam sputtering technique. East German patent No. 149,549 to Hinneberg et al. alludes to multiple ion beam sputtering of semiconductor films using beams of noble gas ions and hydrogen or halogen ions, all at energies of 1 keV to 20 keV. Hinneberg et al. do not mention any attempt to determine or control the hydrogen-semiconductor coordination in deposited films. Their only example uses a single ion beam of mixed sputtering and hydrogen ions.